To create a shape on an existing shape layer, select the shape layer.

To create a shape on a new shape layer with the dimensions of the composition, deselect all layers by pressing F2.

To create a mask, select a layer in the Timeline panel, Layer panel, or Composition panel. To create a mask on a shape layer, you must also click the Tool Creates Mask button in the Tools panel with a shape tool active.

To replace a mask path, select the mask in the Timeline panel, Layer panel, or Composition panel.

To replace a shape path, select the shape path (not the group) in the Composition panel or Timeline panel.